During construction it is often necessary to shore up an excavated earth wall. For example, the laying of pipe within a deep trench typically requires a portable trench shield or trench box to be positioned within the portion of the trench in which a new section of pipe is to be placed. The walls of the trench shield protect the workers from collapsing earth. As each piece of pipe is laid, the trench shield is dragged forwardly within the trench by heavy equipment for placement of the next section of pipe.
It is thus desirable to have a trench shield assembly that is strong, has a high section modulus, and is durable such that it will safely shield workers and withstand handling by heavy equipment.
Various trench shield systems have been developed which include first and second parallel shield walls that are spaced apart by spreader bars. To lighten the weight of the shield walls while providing structural integrity, the shield walls are often of hollow construction.
One such example of a conventional hollow shield wall is disclosed by U.S. Pat. No. 4,345,857 to Krings. An internal frame for the shield wall is constructed from parallel horizontal members that are spaced apart on vertical pipes. An outer skin is then secured to either side of the framework.
Another conventional shield wall is disclosed by U.S. Pat. Nos. 3,992,887 to Fisher. The disclosed shield wall is constructed from a frame having horizontal and vertical members over which metal skins are secured. The inner surfaces of the shield walls carry cylindrical collars that are received within the ends of cylindrical spacer beams, to spread and maintain two opposing shield walls apart from each other.
Yet another conventional hollow shield wall construction is disclosed by U.S. Pat. No. 4,114,383 to Nieber. Metal skins are secured over an inner skeletal framework. The inner surface of the shield walls also include cylindrical collars that are received within the ends of cylindrical spacer beams to spread opposing shield walls apart.
Such conventional hollow shield walls have a drawback in that the outer skins are easily pierced by heavy equipment, allowing the interior framework of the shield wall to fill with soil and water. This substantially adds to the weight of the shield walls, making them difficult to reposition within the trench and to transport to other construction sites. Further, it has been found that the sectional strength of such conventional hollow-skinned shield walls is somewhat less than desirable.
An additional drawback of conventional trench shield systems, such as those disclosed by the Fisher '887 and Nieber '383, is the propensity for heavy equipment to sometimes knock the spacer beam collars off of the inside surfaces of the shield walls. At the least, this necessitates repair of the shield wall; at the worst, it could potentially result in injury.